Mosquito-borne diseases are major public health concerns. Deciphering the interactions between a pathogen and its vector host is crucial for both understanding the evolutionary success of pathogens of human mosquito-borne diseases and devising novel methods to interrupt the pathogen transmission cycle. Our goal is to elucidate the molecular mechanisms of innate immunity during the invasion of pathogens and parasites in mosquitoes. Our research, utilizing transgenesis and reverse genetics, has demonstrated that Toll and IMD signaling pathways regulate the expression of mosquito immune genes and are important in the defense against microorganisms and Plasmodium. Microarray analyses revealed repertoires of immune genes that are affected by Plasmodium, the Dengue virus and depend on Toll, IMD, or JAK/STAT pathways. There is an extraordinary expansion of the genes involved in the melanization pathway-another key pathway of the mosquito immune system. This suggests the presence of separate mechanisms directed specifically toward efficient melanization of invading pathogens. We will test the hypothesis that there are distinct pathways linked to various defense responses, such as tissue and pathogen melanization. We will decipher regulatory networks controlling genes of the immune melanization cascade. Finally, we will investigate the role of microRNAs in these regulatory networks. By analyzing regulation of these genes, we will gain insight into the mechanisms by which the parasite manipulates the host immune system. Accomplishment of the research goals outlined in this proposal will significantly advance our understanding of sophisticated immune mechanisms existing in vector mosquitoes and their involvement in defense against pathogens.